1. Field of the Invention
The present invention is directed to an ultrasound sensor, particularly for shockwave measurements, of the type having comprising a piezoelectric foil, a signal electrode and a shell electrode.
2. Description of the Prior Art
Such ultrasound sensors of the above type are suited both for local pressure measurement and for field measuring in ultrasound fields. The sensors, however, must thereby meet a number of demands. Namely, the sensors must have an adequately high, upper limit frequency, an adequately long service life, particularly when measuring focused shockwaves, and an adequately high sensitivity for the field measurement.
An ultrasound sensor that essentially meets these demands is disclosed in European Application 0 227 985. In this ultrasound sensor, the shell electrode and the signal electrode are arranged spatially separated from the piezoelectric foil, whereby coupling of the alternating charge signal generated by the action of the ultrasound, or shockwaves from the piezoelectric foil onto the electrodes ensues through a liquid. Dependent on whether the liquid is a dielectric liquid or an electrolyte, the signal coupling thereby ensues capacitatively or via the series (intermediate) resistance formed by the liquid. A disadvantage of this known ultrasound sensor is that the sensitivity of the sensor, particularly for sonic field measurements, is not adequate in all cases because of the relatively large distance between the piezoelectric foil and the electrodes. Another disadvantage is that falsifications of the measured results can occur due to non-linear compression properties of the liquid situated between the piezoelectric foil and the electrodes, due to cavitation effects in this liquid and due to positional variations. The presence of liquid between the piezoelectric foil and the electrodes can be avoided when, as disclosed in European Application 0 351 285, electrodes are deposited on both sides of the piezoelectric foil, these electrodes completely covering that surface of the piezoelectric foil to which they are allocated. Based on the teachings of U.S. Pat. No. 4,911,172, moreover, there is the possibility of gluing the piezoelectric foil to an electrode with a thin adhesive layer, whereby the electrical connection ensues by capacitative coupling through the adhesive layer. Both types of sensor, however, have inadequate service lives when charged with high-intensity shockwaves.